This invention relates to a heater control device for an air-fuel ratio sensor which detects an air-fuel ratio of an engine.
At present, an air-fuel ratio sensor is provided at an exhaust system, to accurately control the air-fuel ratio of the sucked mixture of an internal combustion engine. The air-fuel ratio sensor detects an exhaust gas composition which is correlated to the air-fuel ratio, by which a fuel supply quantity is controlled by a feedback control.
In such air-fuel ratio sensor, a heater is provided to heat the sensor element for preventing a deterioration of characteristic of the sensor, and for removing the temperature dependency thereof. Various heater controls are proposed to maintain the sensor element at a constant value of an activation temperature or more. As for the control of this kind, as described in Japanese Unexamined Patent Publication No. 35347/1986, a heater control device is known, in which a bridge circuit including the heater as an element, and the heater is controlled by flowing current so that this bridge is balanced. Explanation will be given to this conventional example, referring to drawings.
FIG. 1 is a total construction diagram of an air-fuel ratio control system including a heating control device of an air-fuel ratio sensor. FIG. 2 is a construction diagram of an air-fuel ratio detecting device of the sensor containing an air-fuel ratio sensor and the conventional heater control device. The air-fuel ratio control system which contains the heater control device of an air-fuel ratio sensor, shown in FIG. 1, is similar to the present invention, mentioned later. In describing the conventional heater control device of the air-fuel ratio sensor, explanation will be given by utilizing FIG. 1, and referring to FIG. 2.
In FIG. 1, a reference numeral 1 designates an engine, 2, an exhaust gas pipe, 3, an air-fuel ratio sensor installed at the exhaust gas pipe 2, 4, an intake air pipe, 5, a throttle valve, 6, a throttle position sensor of the throttle valve 5, 7, an intake air quantity sensor, 8, an engine speed sensor, 9, a battery, and 10, a fuel injection valve.
On the other hand, the air-fuel ratio sensor 3 is connected to the air-fuel ratio detecting device 40, through the harness 21 on the sensor side (lead wire 21l, coupler 21c), and the harness 20 on the air-fuel ratio detecting device side (lead wire 20l, coupler 20c).
Furthermore, a numeral 50 designates an air-fuel ratio control device, which is installed in a driving room as well as the air-fuel ratio detecting device 40. In the drawing, the intake air quantity Q.sub.a the throttle opening degree .theta. and the engine revolution number N.sub.e, which are the state quantities showing a running condition of the engine 1, are detected respectively, by the intake air quantity sensor 7, the throttle valve position sensor 6 and the engine speed sensor 8, and sent to the air-fuel ratio control device 50.
The air-fuel ratio the mixture of the sucked air introduced through the throttle valve 5, and fuel injected by the fuel injection valve 10 in the intake air pipe, is detected by the air-fuel ratio sensor 3 installed at the exhaust gas pipe 2, by using the air-fuel ratio detecting device 40, an output of which is sent from the air-fuel ratio detecting device 40 to the air-fuel ratio control device 50.
As shown in FIG. 2, the air-fuel ratio sensor 3 is composed of the sensor element 31 and the heater 32. The sensor element 31 is composed of the oxygen pump element 31a, the oxygen concentration cell 31b, the exhaust gas diffusion element 31c, and the standard oxygen element 31d.
The temperature of the sensor element 31 is to be maintained at a constant temperature more than an activation temperature even when exhaust gas temperature is changed by the running condition of the engine 1. For that purpose a bridge circuit is formed by the heater resistance Rh of the heater 32 and the resistances R1, R2 and R3 in the air-fuel ratio detecting device 40. In the heater control circuit including this bridge circuit, a balance voltage is detected by the differential amplifier OP1, and controlled by the integral amplifier OP2. The control result is fedback to the buffer OP3, by which the control transistor Tr is driven. The driving current flows in the path of the coupler 20c, the lead wire 201, the coupler 21c, the lead wire 211, the heater 32, the lead wire 212, the coupler 21c, the lead wire 202, the coupler 20c and the resistance R1. The driving current is controlled so that the temperature of the heat resistance R1 becomes a predetermined target value, that is, a predetermined constant temperature.
When the engine 1 is driven, the heater 32 is driven, and the temperature thereof becomes a constant temperature which is determined by the bridge circuit, and the sensor element 31 is activated, the oxygen concentration cell 31b generates an electro motive force V.sub.s which corresponds with an oxygen concentration difference of the exhaust gas diffusing element 31c and the standard oxygen element 31d.
When the electro motive force V.sub.s is controlled by flowing the control current Ip in the oxygen pump element 31a, so that it becomes a predetermined constant voltage V.sub.ref through the differential integral amplifier OP4 in the air-fuel ratio detecting device 40, the control current Ip becomes proportional to the air-fuel ratio.
The control current Ip is detected by the detecting resistance R.sub.o, and is amplified by the differential amplifier OP5, by which the air-fuel ratio output Vout is obtained.
The air-fuel ratio control device 50 calculates the fuel injection quantity which is compatible to the actual intake air quantity, concretely, a valve opening time of the fuel injection valve 9, based on memorized programs and data, so that a predetermined air-fuel ratio is obtained, from the information of the revolution number Ne, the intake air quantity Qa, the throttle valve opening degree .theta., the battery voltage VB and the like. Furthermore, the air-fuel ratio control device 50 controls the fuel injection quantity by a feedback control, by injecting fuel corresponding with the valve opening time from the fuel injection valve 10, so that the air-fuel ratio of the engine 1 becomes the target air-fuel ratio.
In this occasion, since the heater resistance is controlled constant by the heater control device, the temperature of the air-fuel ratio sensor 3 is controlled in a small range of change in spite of the change of the running condition of the engine 1.
However, the heater control device of the conventional air-fuel ratio sensor is constituted as above, the air-fuel ratio detecting device 40 including the heater control device is normally arranged in a driving room. Therefore the air-fuel ratio detecting device 40 has to be connected to the air-fuel ratio sensor 3 through the long harness 20. The influence of the lead wire resistance and the coupler contact resistance of the harness 20 on the heater control temperature is not negligible. Accordingly, in such air-fuel ratio detecting device 40, the balance condition of the bridge circuit, is, from FIG. 2, EQU (Rh+2R1+4Rc)/R1=R2/R3 (1)
where R1 is a resistance of the lead wire 201 between the coupler 20c and the air-fuel ratio detecting device 40, and Rc, the contact resistance of the coupler. Normally the resistance of the lead wire 211 is negligible because the lead wire length is short compared with that of the lead wire 20l.
Generally the heater 32 is of a platinum resistance body. The target control resistance R1 as an operational temperature is set so that a balance condition of the bridge becomes, EQU Rh/R1=R2/R3 (2)
Therefore the actual target resistance value of the heater becomes lower than the set value due to the resistances of the lead wire and the coupler (2R1+4Rc) Therefore the temperature of the air-fuel ratio sensor 3 is lowered by that amount.
The temperature lowering deviation due to this resistance (2R1+4Rc), becomes about 70.degree. C., sensor resistance R1 of the lead wire 201 is about 150 m.OMEGA. since the length of the lead wire is normally about 5 m, the contact resistance of the coupler is several tens m.OMEGA., and the temperature gradient of the heater 32 with respect to the resistance is about 150.degree. C. Therefore, in the domain in which the exhaust temperature of the engine 1 is lower, the temperature of the air-fuel ratio sensor becomes lower than an allowance value, which worsens the exhaust gas and the drivability.
Moreover, when the temperature in the engine room is elevated, and the lead wire resistance R1 is increased, this temperature lowering tendency becomes further significant, which deteriorates the sensors.